重离子碰撞中同位旋自由度输运和对称能约束

重离子核反应是地面实验室研究核物质状态方程的有效手段。当核物质体系 (包括原子核) 中的中子和质子数目差异很大时，核物质状态方程中除了对称核物质贡献之外，对称能项的贡献逐渐变得重要。它反映了核子相互作用势的同位旋矢量部分，与致密星体的性质和恒星核合成等天体物理基本问题，以及远离β稳定线原子核的奇异性质与核素图边界等核物理前沿问题，都紧密相关。然而，迄今为止，对称能对密度的依赖行为尚未很精确地约束。因此，对称能的研究成为当前国际上主要的中能核物理实验室和天体观测装置的主要物理目标之一。本文简单评述了这一领域的实验进展，并介绍了基于国内的大科学装置，即兰州重离子研究装置(HIRFL)开展费米能区重离子反应与对称能实验研究的一些进展。实验结果表明，这一能区的同位旋自由度输运时标和具体的物理过程相关，同位旋漂移效应可能持续到反应晚期。由于同位旋效应的长时间积累，轻粒子同位旋的角度分布可以用来约束对称能的密度依赖，在饱和点附近对称能值为$ S\!=\!28.3 $ MeV的条件下，其斜率参数约束在$L\!=\!33\!\sim\! 61\,\mathrm{M}\mathrm{e}\mathrm{V}$的区间，置信度水平为95%。借助一套具有裂变碎片和带电粒子符合测量能力的高性能重离子反应测量谱仪，可以测量具有同位素分辨的带电粒子小角关联函数，从而给出费米能区重离子反应中同位旋驰豫的时标。最后，介绍了氘核的同位旋矢量极化效应，该效应可能提供一种全新的约束对称能密度依赖的途径。

Transport of Isospin Degree of Freedom in Heavy Ion Reactions and the Constraint of Symmetry Energy

Heavy ion reactions provide an effective tool to study the nuclear equation of state (EOS) in terristial laborartory. When the number of neutrons differs largely with the number of protons in a nuclear system or nuclei, the main contribution to the nuclear EOS comes from the symmetry energy term. The nuclear symmetry energy, reflecting the isovector sector of the nucleon-nucleon potential, is closely relevant to the structural properties of dense object and the merging process in stellar environments, as well as to the exotic properties of nuclei and the location of the board of nuclear chart. However, the density dependence of the nuclear symmetry energy is the most unknown ingredient in the properties of nuclear matter so far. Thus the investigation of nuclear symmetry energy in wide density range becomes the main frontiers in many world-level nuclear laboratories and astrophyics observatories. In this article, we review briefly the experimental progress in this field. Particularly the experimental studies on the transport of the isospin degree of freedom in heavy ion reactions and the constraint of nuclear symmetry energy based on the Heavy Ion Research Facity at Lanzhou (HIRFL) are introduced. It has been shown that the isospin drift process persists to the late stage of the reaction, indicating that the isospin transport time scale may depend on the physics process under investigation. Due to the long-time accumulation of isospin effects, the angular distribution of the isospin concentration of the light particles in a wide range of angle is a sensitive probe of nuclear symmetry energy. With ${S}\!=\!28.3\,\mathrm{M}\mathrm{e}\mathrm{V}$ fixed at saturation point, the slope of the symmetry energy depending on density is contrained in the range of $L\!=\!33\!\sim\!61\,\mathrm{M}\mathrm{e}\mathrm{V}$ at $ \mathrm{C}\mathrm{L}\!=\!95$%. Using a Compact Spectrometer for Havy Ion Experiment (CSHINE) with ability to achieve isotopic particle identification and fission event reconstruction, the small angle correlation function of the isotope-resolved light charged particles can be measured to derive the time scale of isospin relaxation in heavy ion reactions at Fermi energies. In the last section of the article, the isovector orientation effect of the polarized deuteron beam scattering off heavy target is introduced, which offers a novel means to constrain the nuclear symmetry energy below saturation density.